Today, optical fiber systems are in widespread use in both public and private telephone and data networks. In the early stages of optical fiber networks, however, deployment was limited to high-revenue-generating applications. This limited deployment was due to communications-equipment manufacturers making network components using unique, proprietary architectures. The result of which, of course, was that the network components from one manufacturer did not work with other manufacturers' network components. An operating company implementing an early optical fiber network had to purchase most, if not all, of its network components from one manufacturer.
In order to provide inter-operability among components from the various manufacturers (and thus lower costs to the operating companies), Bellcore established a standard for connecting one optical fiber component or system to another. That standard is officially named the “Synchronous Optical Network,” but is more commonly called “SONET.” The international version of the standard is officially named the “Synchronous Digital Hierarchy,” but it is more commonly called “SDH.”
Although differences exist between SONET and SDH, those differences are mostly in terminology. In most respects, the two standards are the same and, therefore, virtually all equipment that complies with either the SONET standard or the SDH standard also complies with the other. Therefore, for the purposes of this specification, the SONET standard and the SDH standard shall be considered interchangeable and the acronym/initialism “SONET/SDH” shall be defined as either the Synchronous Optical Network standard or the Synchronous Digital Hierarchy standard, or both.
The basic SONET/SDH signal is defined as a Synchronous Transport Signal level 1 (STS-1) frame. An STS-1 frame is an 810-byte data packet that comprises transport overhead (the information required to maintain communication) and payload (the data itself). For the purposes of this specification, a “STS-N” is defined to comprise N STS-1s. For example, an STS-768 comprises the data from 768 STS-1s plus the overhead of the STS-768. Furthermore, for the purposes of this specification, an “STS-N frame” is defined to comprise N STS-1 frames of data and the overhead of the STS-N frame. For example, an STS-768 frame comprises 768 STS-1 frames.
Also, for the purposes of this specification, a “SONET/SDH network” is defined as two or more nodes and transmission facilities (e.g., optical fibers, repeaters, etc.) that connect the nodes. FIG. 1 illustrates a block diagram of a SONET/SDH network 10 in the form of a “ring,” as is well known in the art. In this elementary example, there are three nodes 12, 14 and 16 connected in a closed loop by a pair of optical-transmission facilities 18 and 20.
For the purposes of this specification, a “node” is defined as a network element in a telecommunications network that;                i. originates and/or terminates digital signals, or        ii. that digitally cross-connects digital signals, or both i and ii.        
In this example, each of nodes 12, 14, and 16 is connected to a plurality of sources and/or destinations for data traffic, which are well known in the art as “tributaries.” Node 12 originates/terminates traffic between network 10 and tributaries 32, node 14 originates/terminates traffic between network 10 and tributaries 34 and node 16 originates/terminates traffic between network 10 and tributaries 36. Each of nodes 12, 14, and 16 receives data from one or more of its respective tributaries 32, 34, and 36 at an STS-N rate, multiplexes the data to the data rate of the ring (which is, by definition, higher than the data rate of the tributaries), and transmits the data around SONET/SDH ring 10. Simultaneously, each of nodes 12, 14, and 16 receives data from the SONET/SDH ring 10, demultiplexes the data to the data rate of the destination tributary and sends the data on the tributary.
As stated above, each of nodes 12, 14, and 16 in SONET/SDH ring 10 is connected to the next node by a pair of optical transmission facilities 18 and 20. In normal operation, each node transmits STS-N frames around ring 10 either counterclockwise on optical transmission facilities 18 or clockwise on optical transmission facilities 20.
When a discontinuity or failure occurs in a SONET/SDH ring, the affected traffic is re-routed around the discontinuity in accordance with a procedure called “automatic protection switching.” In order to implement automatic protection switching, each SONET/SDH ring defines a distinct address space and a unique address (or “Node ID”) that uniquely identifies each node within the network. The current SONET/SDH standard specifies that addresses in the address space of a SONET/SDH ring are carried in the K1 and K2 bytes in the line overhead of an STS-N frame.
The K1 and K2 bytes comprise:
K1 Byte:Bits 1–4:Type of automatic protection switch request(lock out of automatic protection switching,forced switch, signal failure, signaldegradation, manual switch, etc.).Bits 5–8:The destination Node ID of the automaticprotection switch message.K2 Byte:Bits 1–4:Source Node ID of the automatic protectionswitch message.Bit 5:Indication of automatic protection switching(short or long path).Bits 6–8:Mode of operation (Line alarm indicationsignal, line remote defect indication, etc.).
In the example of FIG. 1, nodes 12, 14, and 16 have Node ID's according to Table 1:
TABLE 1Node Addresses for SONET/SDH Ring 10NodeSONET/SDH Ring 10 Node IDNode 120Node 141Node 162
For purposes of understanding automatic protection switching in the prior art, assume that node 12 is receiving traffic on one or more tributaries 32 destined for node 14's tributaries 34. Furthermore, assume that node 14 detects a fault or failure on the optical transmission facility 18 between node 12 and node 14. Node 14 notifies both node 12 and node 16. To this end, node 14 populates an STS-N frame overhead K1 and K2 bytes for node 12 as follows:
K1:bits 1–4automatic protection switch request.bits 5–8:the Node ID of node 12 (“0” in this example).K2:bits 1–4:its own Node ID (“1” in this example).bit 5:short path.bits 6–8:the remote defect indication (“RDI”).
Node 14 sends the STS-N frame in the clockwise 20 direction.
Node 14 notifies node 16 by populating an STS-N frame overhead K1 and K2 bytes as follows:
K1:bits 1–4automatic protection switch request.bits 5–8:the Node ID of node 12 (“0” in this example).K2:bits 1–4:its own Node ID (“1” in this example).bit 5:long path.bits 6–8:the bridged and switched state.
Node 14 sends the STS-N frame in the counterclockwise 18 direction.
Node 12 receives the K1 and K2 bytes from the STS-N frame on clockwise optical transmission facility 20. Node 12 reacts to the K1 and K2 bytes by discontinuing transmission on optical transmission facility 18, and switching to clockwise optical transmission facility 20. In the counterclockwise direction, node 16 reads the K1 and K2 bytes, notes that its own Node ID, “2,” is not in the K2 byte, and does not change the K1 and K2 bytes (“pass through mode”).
For the purposes of this specification, the term “short path” is defined as the path between the two nodes adjacent to the failed span that includes the failed span, and the term “long path” is defined as the path between the two nodes adjacent to the failed span that does not include the failed span. Therefore, when a discontinuity or failure occurs in a SONET/SDH ring, the affected traffic is re-routed from the short path to the long path.
The example of FIG. 1 illustrates only three nodes in network 10. More nodes are usually present, as is well known in the art. A problem in the art exists, however, because the SONET/SDH standard limits the maximum number of source and destination nodes in it definition of K1 and K2 bytes to four bits, or 16, thus limiting the size and flexibility of SONET/SDH rings. As demand for data traffic increases, this limitation on the number of nodes in a ring requires that, after 16 nodes are equipped in a network, an entire new network must be added at considerable expense.
Furthermore, the number of tributaries in one location along the ring can require more tributaries that one node can support. In the prior art, this scenario requires a new node to be defined in the address space of the network. Therefore, it is an object of this invention to provide a means to increase the number of nodes in a SONET/SDH network beyond current limitations without changing the SONET/SDH standard or modifying the existing nodes in the ring.